Biological materials such as semen are commonly preserved by freezing at cryogenic temperatures. The containers which are used for the biological specimens must provide a completely closed and sealed environment to prevent leakage of gases and liquids into the container at cryogenic temperatures. Specimen containers may, for example, be plastic or glass ampoules. The specimen-containing ampoules are cooled below freezing and eventually are transferred to permanent ultra-low temperature storage facilities.
The rate of cooling of a biological specimen during the freezing process may be critical to the ability of the specimen to display viability when eventually thawed. Freezing procedures in the past have employed a method in which plastic or glass specimen containers simply are suspended in a cold vapor for a predetermined period of time to allow the specimens to freeze, and the containers then are plunged into a liquid cryogen such as liquid nitrogen. This relatively simple procedure does not take into account aspects of initiation of ice nucleation ("seeding"), nor is compensation made for the supercooling phenomenon. The phenomenon of supercooling involves the ability of a liquid to be cooled well below its normal freezing point before undergoing a phase change. When freezing occurs, the temperature of the specimen may rapidly increase to approximately the normal freezing point upon liberation of the heat of fusion, following which the temperature of the solid frozen specimen decreases. For example, if a standard 320 milliosmol solution contained in a sealed plastic container were to be permitted to supercool below its normal freezing point as its temperature was gradually lowered, freezing may occur at a temperature of from about -14.degree. to about -18.degree. Celsius. At the moment of the phase change, the sample temperature may dramatically rise toward -0.5 to -1.degree. C., before falling again. The resulting fluctuation of temperatures in the vicinity of the freezing point may be very harmful to many biological specimens, and desirably should be avoided as much as possible. Temperature fluctuations have a tendency to harm such biological specimens because it causes alternate cell hydration and cell dehydration through cell wall osmosis. The corresponding water movement weakens the cell wall and diminishes the integrity of the specimen. Accordingly, it is desired to minimize temperature fluctuations due to supercooling to in turn decrease the damage which is done to the sample due to the freezing process.